Aqueous polyvinyl alcohol stabilized polymer emulsions comprising ethylene and vinyl esters, particularly vinyl acetate, as the major monomers find widespread applications as adhesives in the areas of packaging and converting and for vinyl laminating. For these uses, the adhesives are prepared and employed in emulsion form and, on removal of the aqueous medium, will cure or harden at room temperature to form a bond which is desirably characterized by high strength and resistance to heat, humidity and water.
The adhesives are particularly useful to provide laminates of woven and non-woven fabrics where the fabric itself is of cotton, polyolefin, polyester, polyamide (nylon), etc.; coated and uncoated paper and paperboard; films such as polyvinylidene chloride (PVDC), polyester, PVDC coated polyester, oriented and non-orientated polyethylene, polypropylene and polystyrene films, metallic foils and metallized films; and flexible cellular material such, for example, as polyurethane foams or sponge rubber. The laminates can be made of similar or dissimilar laminae and are useful in a wide variety of end-use applications including, for example, disposable flexible packaging, labelling, general packaging, cigarettes, envelopes, graphic arts and industrial uses such, for example, as weather stripping and electrical insulation.
Aqueous emulsion adhesives should be capable of providing an adhesive that can exhibit excellent adhesion to all types of substrates, excellent hot green strength, initial adhesion, resistance to thermal creep, heat resistance, high temperature bond strength, resistance to water, and low temperature adhesion. New technological developments in related fields such as plastics and high speed adhering machines, along with increasing needs for superior adhesion under various stringent conditions call for improved high performance adhesives. Typical examples are the widespread use of hydrophobic materials as substrates (difficult to bond substrates) such as metallized plastic films, water-repellent paper, resin coated papers, plastic to wood, etc., which necessitates the invention of novel adhesives also.
For example, the disposable wipe, bag and envelop industries would certainly benefit from an improved version of the existing aqueous based polymer adhesive base as it will reduce the need to formulate with plasticizers and tackifiers. Improving the polymer adhesive base will also lead to decreased levels of expensive plasticizers used, thereby lowering the cost.
In the case of microwaveable adhesives, better adhesion and heat resistance in a polymer base reduces the need to formulate with crosslinkers and can also increase shelf-life. Excessive crosslinking may lead to generation of a hard polymer which generally sacrifices adhesion to the substrate.
In the case of vinyl laminating, too soft a polymer results in poor heat resistance and poor bonds. At the same time, too hard a polymer will lead to poor adhesion characteristics. The existing latex based polymer bases generally suffer from low hot green strengths, poor heat resistance, and need further formulation to enhance specific adhesion characteristics.
This invention relates to an ethylene vinyl acetate copolymer emulsion adhesive that addresses many of the drawbacks of existing aqueous based adhesives such as those mentioned above. Such adhesives can be produced on an industrial scale without many changes from the currently used equipment.